Cellular and tumor immune responses were examined with different assays. T-cell-mediated cytotoxicity against tumor-associated antigens on intact tumor cells does not require similarity at the major histocompatibility (H-2) complex (MHC). T cell-tumor cell interaction to produce migration inhibition factor (MIF) does not require H-2 compatibility. Soluble antigen can stimulate MIF production only with macrophages sharing part of the MHC with the responding lymphocytes, but when intact tumor cells are used as antigen, macrophages are not required. Different subpopulations of T lymphocytes produce MIF, depending on whether the antigen is soluble or intact. Syngeneic or allogeneic tumor cells can induce secondary cytotoxic responses against syngeneic targets. In vitro secondary cytotoxic responses require proliferation. Macrophages in tumor-bearing animals are activated to kill tumor cells. Adherent cells from peritoneal exudate, spleen and tumor can nonspecifically lyse many tumor targets. Activation of macrophages requires T cells with some inducers while in other circumstances (as in nude mice) macrophage activation can occur without T lymphocytes. Effector cells that mediate natural killer activity and those that mediate antibody-dependent cytotoxicity against tumor cells have similar characteristics and may belong to the same subpopulation of cells. BIBLIOGRAPHIC REFERENCES: Holden, H.T., Oldham, R.K., Ortaldo, J.R. and Herberman, R.B.: Standardization of chromium-51 release, cell-mediated cytotoxicity assay: Cryopreservation of mouse effector and target cells. J. Natl. Cancer Inst. 58: 611-622, 1977. Holden, H.T., Landolfo, S. and Herberman, R.B.: T-cell-dependent reactivity against tumor-associated antigens on allogeneic target cells. In Transplantation Proceedings. New York, Grune and Stratton, 1977, Vol. IX, pp. 1149-1152.